The present invention relates to a video signal processing apparatus and, particularly, to a video signal processing apparatus having an image quality adjusting means for automatically regulating an image quality and is suitable to be applied to a video tape recorder (referred to as VTR hereinafter).
A home VTR is well known as disclosed in "Video Technology Handbook", edited by monthly journal, "Audio Video", Editing Division, pages 73 to 87, which will be described in brief with reference to FIGS. 21 and 22. In the following description, the VTR is of VHS system corresponding to NTSC signal for convenience.
FIG. 21 is a block circuit diagram of an example of a conventional reproduction system of a home VTR. In FIG. 21, an FM luminance signal and a low frequency color signal recorded on a magnetic tape 1 are detected by a magnetic head 2 and, after being amplified by a reproduction amplifier 3, supplied to a low pass filter (LPF) 4 and a high pass filter (HPF) 7. Only the low frequency color signal which is spread with a center frequency of 629 kHz passes selectively through the LPF 4 and, after being converted by a frequency converter 5 into a high frequency color signal having center frequency of 3.58 MHz and passed through a band pass filter (BPF) 6, supplied to an input of an adder 11. On the other hand, only the FM luminance signal whose carrier frequency is set to 3.4 MHz to 4.4 MHz passes selectively through the HPF 7 and, after being demodulated by an FM demodulator 8 to a video luminance signal and passed through a LPF 9, supplied to an image quality adjusting circuit 10. The image quality adjusting circuit 10 has a function of reducing noise of a reproduced image or emphasizing contour information thereof by changing gain vs. frequency characteristics thereof for an input signal. An output of the image quality adjusting circuit 10 is supplied to another input of the adder 11 and added to the high frequency color signal, resulting in a reproduced video signal at a terminal 12. Therefore usually characteristics of the image quality adjusting circuit 10 therefore are changed by a voltage control. Therefor conventionally the characteristics of the image quality adjusting circuit therefore are changed manually by means of a switch or a volume provided on a front portion of the VTR for making the control voltage variable.
FIG. 22 is a block circuit diagram of an example of a conventional recording system of a home VTR. In FIG. 22, a video signal supplied to an input terminal 36 is supplied to a BPF 37, a LPF 40 and a terminal 47. The terminal 47 is an EE output terminal for monitoring the input signal and the inputted video signal appears at the EE terminal 47 substantially as it is. A high frequency color signal of the input video signal, which is centered at 3.58 MHz, passes selectively through the BPF 37 and, after being converted by a frequency converter 38 into a low frequency color signal and passed through a LPF 39, is supplied to a recording amplifier 43. On the other hand, a video luminance signal passes selectively through the LPF 40 and, after being modulated to an FM luminance signal by means of an FM modulator 41 and passed through a HPF 42, is supplied to the recording amplifier 43. In the recording amplifier 43, the FM luminance signal and the low frequency chroma signal are mixed, suitably amplified and recorded on a magnetic tape 45 through a magnetic head 44. As is clear from the above description, there is no automatic image quality adjusting mechanism for the EE output signal and the recording signal provided in the conventional apparatus.
In the conventional technique mentioned above, image quality adjusting for the reproduced signal is performed manually. Therefore, in order to enjoy good image quality continuously, it is necessary to make optimum adjustments every time the reproducing tape is changed, according to a variation of signal to noise (S/N) ratio due to degradation of magnetic tape. Further, even during a continuous reproduction of a same tape, optimum image quality adjustment changes every scene since noticeability of noise varies with change of brightness of the reproduced image. It is very difficult to finely adjust the image quality manually in response to such variation. Further, since there is no automatic image quality adjusting means provided for the recording signal and the EE signal, it is impossible to adjust the image quality according to brightness of an image and/or S/N ratio of the input signal.
FIG. 23 shows another conventional reproduction system; shown in pages 76-87 of the aforementioned article. The conventional system in FIG. 23 differs from that shown in FIG. 21 in that the image quality regulation means takes in form of a noise canceller 18 and that it further includes an auto tracking system including an FM signal amplitude detector 22, an auto tracking micro computer 24, a system control micro computer 25, a capstan servo circuit 26 and a capstan motor 27.
In FIG. 23, an amplitude of the FM signal from the reproducing amplifier 3 is converted into DC voltage information corresponding to the signal amplitude by the FM signal amplitude detector 22 and supplied to the auto tracking micro computer 24. The auto tracking micro computer 24 determines, on the basis of the DC potential information, an optimum tracking phase so that the amplitude of the reproduced FM signal becomes maximum and sends a result to the system control micro computer 25. The system control micro computer 25 continuously manages the VTR and, when an auto tracking operation is required, for example, immediately after a tape is inserted, sends the information from the auto tracking micro computer 24 to the capstan servo circuit 26 which responds thereto to send a control signal to the capstan motor 27 to automatically control a tape running speed such that the magnetic head 2 correctly traces the recording track pattern recorded on the magnetic tape 1.
As to the noise canceller 18 shown in FIG. 23, it is not always necessary for a home VTR to reproduce a signal with the same fidelity as required for a broadcasting VTR so long as it is visually acceptable. Therefore, the noise canceller 18 having such circuit construction as shown in FIG. 24 has been used conventionally. In FIG. 24, the noise canceller 18 comprises a filter 120 having an input connected to an input terminal 119, an amplifier 121, a limiter 122, a level matching circuit 123, all of which are connected in series in the order, and a subtractor 124 having a minus input connected to an output of the level matching circuit 123, a plus input connected to the input terminal 119 and an output terminal 125. The reproduced video signal from the LPF 9 is supplied through the input terminal 119 to the plus input of the subtractor. 124 and the input of the filter 120. Since the reproduced video signal inputted to the input terminal 119 is obtained by the FM demodulation, high frequency component thereof includes larger noise due to the nature of the so-called triangle noise which is unique for FM processing. In order to remove this noise, a high pass filter is used as the filter 120. The high frequency component passed through the filter 120 is amplified suitably by the amplifier 121 and then amplitude-limited by the limiter 122. The threshold value of the limiter 122 is usually set to a value slightly larger than noise amplitude for a video signal having usual signal to noise ratio. Therefore, in this case, the noise component is supplied to the level matching circuit 123 without amplitude limitation. On the other hand, almost all information of the signal component is lost by means of the amplitude limiting effect of the limiter 122 and therefore the level matching circuit 123 receives almost no signal. The output of the level matching circuit 123 is connected to the minus input of the subtractor 124, as mentioned.
It is possible to change an amount of noise component to be substracted from the video input supplied to the plus input of the subtractor 124 by a level setting of the level matching circuit 123. Although, in the subtractor 124, a small amount of the high frequency signal component is subtracted from the input video signal, a waveform degradation resulting therefrom is as small as allowable for the home VTR.
The level matching circuit 123 may have a circuit construction shown in FIG. 25, which includes a resister 127 connected at one end to an input terminal 134 and at the other end to an output terminal 135 and a series connection of a grounding resister 128 and a capacitor 132.
Assuming that the resister 128 has a value R, the level setting is fixed by the value R thereof.
FIG. 26 shows characteristics curves showing a variation of S/N ratio of luminance signal when R is changed, with the value of R (K.OMEGA.) in abscissa and S/N ratio of value of luminance signal in ordinate. A characteristic curve I is obtained when the amplitude of FM signal is 420 mVp-p, a curve II 280 mVp-p and a curve III 200 mVp-p. The reduction of reproduced FM signal amplitude is caused by narrow width of the reproducing magnetic head, tracking deviation during reproduction or demagnetization of the magnetic tape due to-repetitive reproductions. In any case, reproduced FM signal carrier vs. noise ratio is reduced and therefore, S/N ratio of the video signal obtained by demodulation becomes small. In general, the larger the S/N is; the better the reproduction. Among the three curves in FIG. 26, the curve I exhibits the best S/N ratio and the curve III is the worst.
In the case of the curve I, the amount of noise component imposed on the reproduced video signal at the input terminal 19 is small and is not limited by the limiter 22. When R is gradually increased, noise to be subtracted from the input video signal becomes gradually large, enhancing the noise cancelling effect. When the noise included in the reproduced video signal. and the noise amount outputted from the level matching circuit 123 becomes equal, the maximum S/N value is obtained at a point b. In the case of the curve I, it is usual to set the R value to one corresponding to the point b. At a point a on the same curve, the reproducibility of high frequency component of a minute signal whose amplitude is close to that of noise may be better than that at the point b although the S/N ratio is slightly lowered. With R larger than that at the point b, the noise component in reverse phase increases gradually resulting in reduction of S/N ratio since the output of the level matching circuit 123 becomes larger compared with that of the video signal.
In the case of the curve II, the noise component of the reproduced video signal is large and therefore its amplitude of the noise component is limited by the limiter 122. Therefore, in order to coincide the noise amount from the level matching circuit 123 with the noise amount in the reproduced video signal, it is necessary to reduce the attenuation by the R in the level matching circuit 123 compared with the case of the curve I. Therefore, R with which the S/N value becomes maximum becomes larger than that in the case of the curve I.
In the case of the curve III, the above matter concerned to the curve II is further enhanced.
Assuming that the R is initially set to a value corresponding to the point a for reproduction of a signal whose FM signal amplitude is large, the value of the R corresponds to a point c in the curve III for a signal whose FM signal amplitude is small and a resultant S/N ratio of image is visually bad. Therefore, it is preferably to make the R automatically changeable correspondingly to a state of a reproduced signal.